An intercomparison for continuous monitors that measure thoron (220Rn) concentration was carried outusing a 220Rn chamber of National Institute of Radiological Sciences (NIRS), Japan; eleven 220Rn monitors(four types) from nine laboratories were evaluated. The 220Rn detection principle was the same for theeleven instruments and one reference instrument, which were commercially available silicon semiconductor detectors using an electrostatic collection method. The intercomparison results showed that there was a negative deviation of more than 30% in measured 220Rn concentrations given by the laboratories relative to the reference values, which were obtained by making a decay-correction during the travel of 220Rn through the sampling assembly (sample tube, filter and drying unit) and using a calibration factor. In order to elucidate the reason for this and then to investigate factors that affect the 220Rn concentration measured with the monitors. As a result, it was necessary to make the decay-correction, in particular, when using a drying unit with a large inner volume and to use the calibration factor in order to better estimate the 220Rn concentration. It was also found to be better to determine a calibration factor inherent to an individual monitor, because the calibration factor values ranged from 0.75 to 2.32, depending on the flow rate of the monitor pump (0.37–1.02 L min1). It was concluded from this study that a periodical calibration of the monitor and a check of the monitor flow rate during measurements are necessary to maintain a consistent quality level of the 220Rn measurement.